Fine Mapping Soybean Aphid Resistance Gene rag4 in Soybean (Glycine max L.)

First recorded in the year 2000, the soybean aphid (Aphis glycines Matsumura) has been a serious pest for soybean production in the north-central United States and can cause yield loss as high as 40% annually. As development of resistant cultivars to be the best option to control the deadly pest, the research efforts have been done to identify several soybean aphid resistance QTLs in the soybean breeding program at Michigan State University. Among them, the QTL underlying soybean aphid resistance gene rag4 was mapped to the soybean chromosome 13 between Satt348 and Satt649 (two SSR markers separated by 7.6 mbp) interval (Zhang et al. 2009). Objectives of this study are to fine-map the rag4 gene onto soybean genome. In the first step of this study, we located the QTL on chromosome 13 between customer-designed SNP markers MSUSNP13-29 and MSUSNP13-33 (310 Kbp) interval using two recombinant inbred lines (RILs) populations. The QTL explained 22.7% and 20.8% phenotypic variation with -0.53 and -0.51 additive effects under field cage (Summer 2011) and greenhouse (Spring 2012) conditions, respectively. In the second step, fine mapping of the region was done by screening recombination break points in eight residual heterozygous lines (RHLs) and two BC₃F₂ populations of a total of 2619 lines. Based on the results, the rag4 interval was narrowed down to a 39.3 kb segment between SNP markers MSUSNP13-5 and MSUSNP13-7. The information gained in this study will be very useful in the marker assisted-selection and to perform the positional cloning.